The present invention relates to a non-volatile memory device and a method of verifying a program operation in the same.
Recently, the demand has increased for a non-volatile memory device which electrically programs and erases data, and does not require a refresh function of periodically rewriting data.
The non-volatile memory device performs a program operation and an erase operation by changing a threshold voltage of a memory cell through moving of electrons by high electric field applied to a thin oxide film.
The non-volatile memory device includes generally a memory cell array in which memory cells for storing data are disposed in a matrix, and a page buffer for programming data in a specific memory cell of the memory cell array or reading data from a certain memory cell.
The page buffer has a pair of bit lines connected to a corresponding memory cell, a register for storing temporarily data to be programmed in the memory cell array or data read from a given memory cell, a sensing node for sensing voltage level of a specific bit line or a certain register, and a bit line select circuit for controlling connection of the bit line and the sensing node.
Increased storage capacity of the non-volatile memory device has also been required, and so a method of programming a multi level cell has been developed.
In the above method, a given memory cell has at least two different threshold voltage distributions in accordance with one program operation.
For example, in a method of programming a multi level cell having 2 bits, four threshold voltage distributions are generated. In this case, a program pulse is applied once in accordance with an incremental step pulse program ISPP method, and then two or three verifying operations are performed.
That is, two or three verifying operations are performed when a most significant bit MSB program is performed after a least significant bit LSB program is finished. Accordingly, the time for the verifying operation is increased.